1. Field of the Invention
The present invention relates to a high-efficiency solar cell which scatters incident light so as to increase the light absorption in an active layer.
2. Related Background Art
An example of methods of improving the efficiency of solar cells which use a light reflecting substrate is the method of forming a roughened light reflecting surface having unevenness so as to increase the length of the optical path of light having low absorption wavelengths. This method is disclosed in, for example, U.S. Pat. No. 4,126,150 (assignee, RCA) and Japanese Patent Laid-Open No. 56-152276 (assignee, Teijin). The optical effects of a roughened substrate are described in Japanese Patent Laid-Open No. 59-104185 (assignee, Exxon Research and Engineering Company).
Methods of forming an uneven reflecting surface include the wet etching method disclosed in Japanese Patent Laid-Open No. 54-153588 (assignee, National Patent Development Corporation), the sandblasting and co-evaporation methods disclosed in Japanese Patent Laid-Open No. 58-159383 (assignee, Energy Conversion Devices), and the DC electrolytic etching or chemical etching methods for roughening an aluminum surface disclosed in Japanese Patent Laid-Open No. 59-14682 (assignee, Denkai-haku Kogyo).
Japanese Patent Laid-Open No. 63-169769 also discloses an example in which p-type, i-type, and n-type layers are laminated on a transparent electrode formed on a diamond thin film to form a solar cell.
The above-described conventional methods of forming a substrate having an uneven surface produce various problems when a solar cell is formed.
For example, in the DC electrolytic etching method or the chemical etching method using an etching solution, since etching residue is produced on the surface of a substrate, if a semiconductor layer is deposited on the substrate, the etching residue diffuses into the semiconductor layer and thus causes deterioration in the characteristics of the solar cell formed thereon. The etching method also has the problem that since the etching residue has poor adhesion to the substrate, the semiconductor layer deposited on the substrate is easily separated therefrom, and thus a solar cell having uniform characteristics cannot be easily formed.
In the sandblasting method, since the unevenness is formed by spraying fine particles on a substrate, the fine particles remain on the substrate and sometimes serve as nuclei of abnormal growth of a semiconductor layer. This method also has the problem that since the fine particles have poor adhesion to the substrate, the semiconductor layer deposited on the substrate is easily separated. The sandblasting method also produces a distortion of the substrate because unevenness is formed by spraying fine particles on the surface of the substrate. Since the distortion of the substrate is relieved with the passage of time, the semiconductor layer deposited thereon is sometimes distorted and is thus separated, resulting in deterioration of the electrical characteristics.
In addition, when unevenness is formed on the surface of a substrate by the co-evaporation method, there are the problems that nonuniformity is easily produced in the deposited film because at least two materials are simultaneously deposited, and that nonuniformity is easily produced in the characteristics of the solar cell formed by depositing a semiconductor layer on a deposited film.
In the example described in Japanese Patent Laid-Open No. 63-169769 in which a diamond thin film is used as a substrate of a solar cell, a semiconductor layer is deposited on a transparent electrode formed on the diamond thin film to form a solar cell. In such a layer structure, since the diamond thin film is used as a high-resistance substrate in place of a glass substrate, the transparent electrode is provided on the diamond thin film. This conventional example does not make use of the good adhesion between the diamond thin film and the semiconductor layer and has the problem that the constituent elements of the transparent electrode diffuse into the semiconductor layer.